The present invention generally relates to controlling the temperature of a device and, more particularly, to controlling the temperature of a device using a fluidic closed loop cooling system robust against fluid accumulation and capable of fast startup and operation in high temperature and cryogenic temperature ranges, for use primarily in aerospace, electronic, and military applications.
Two types of closed loop cooling systems are capillary pumped loops (xe2x80x9cCPLxe2x80x9d) and loop heat pipes (xe2x80x9cLHPxe2x80x9d). Both are passive heat transport systems and contain no mechanical moving parts. Both the CPL and the LHP are designed to use a fluid to transport waste heat from a controlled device over a long distance and reject it to a heat sink. These systems transfer heat by taking advantage of the latent heat of evaporation, where the heat is absorbed via evaporation and taken out of the system at a sink location where the fluid is condensed. Fluid circulation in both CPL and LHP systems is accomplished entirely by capillary action developed in the ultra-fine pore wicks of the capillary pumps.
The maximum heat transfer capacity of a systems is determined by the capillary limit of the wick. The capillary limit is maximum pressure that a wick can sustain, which is a function of the wick""s pore size and the surface tension of the working fluid. As long as the pressure drop in the system is below the capillary limit, the loops will continue to operate. If the system pressure drop exceeds the capillary limit, vapor will be pushed through the wick structure and block off the incoming liquid, thus causing the wick to dry out or xe2x80x9cdeprime.xe2x80x9d
Both the CPL and LHP consist of an ECP, a condenser, a reservoir, and vapor and liquid transport lines. Basic operational principles of a CPL and a LHP are very similar: (i) waste heat from a heat source conducts through the ECP body to vaporize liquid on the ECP wick""s outer surface, (ii) generated vapor flows in the vapor line to the condenser where heat is removed to condense the vapor back to liquid, and finally (iii) the condensed liquid returns to the ECP in the liquid line to complete the cycle. CPLs are limited by their inability to tolerate vapor in the pump core and have tedious and time-consuming start-up procedures. LHPs are capable of only limited system temperature regulation, but this feature is usually difficult to achieve.
Accordingly, there is a need for a highly reliable heat transport system that is capable of fine temperature control for aerospace and electrical applications. There is a further need for a closed system passive heat transport device that is capable of fast system startup. There is a further need for a closed system passive heat transport device that can prevent vapor accumulation in the system reservoir. Additionally, there is a further need for a closed system passive heat transport device that can operate over a wide temperature range, ranging from cryogenic temperatures to temperatures in excess of 600 degrees Celsius.
According to the present invention, an advanced loop heat pipe (xe2x80x9cALHPxe2x80x9d) is provided. The ALHP is a capillary device capable of transporting a large amount of waste heat over a long distance and rejecting it to a heat sink. The ALHP can start, stop, and re-start at any time (xe2x80x9cturnkey startupxe2x80x9d), provide fine temperature regulation, and operate at cryogenic temperatures without requiring a cooling shield for the return liquid. Furthermore, by selecting a proper working fluid, the ALHP can operate in high temperature and cryogenic temperature ranges.
The ALHP combines the advantageous attributes of both CPLs and LHPs without inheriting operational shortcomings of either one. It starts up quickly and operates reliably like a LHP and also tightly controls the loop operating temperature like a CPL. In addition, the ALHP operates at temperatures far below the surrounding temperature making passive flexible cryocooling possible.
Tight temperature control is accomplished in the ALHP by regulating the mass flow rate of the auxiliary pump (xe2x80x9cAPxe2x80x9d) to maintain the loop temperature at a desired level. The procedures to regulate the AP mass flow rate depend on the type of pump used as the AP. For example, if the AP is a capillary pump, then its mass flow rate is directly proportional to the heater power applied to it. In other words, by increasing or decreasing the AP heater power, the mass flow rate generated by the AP increases/decreases accordingly. If the AP is a mechanical pump, adjusting the pump speed regulates its mass flow rate and thereby controls the loop temperature to a desired level. Or if the AP is an electro-hydrodynamic (xe2x80x9cEHDxe2x80x9d) pump, regulating the applied voltage to the pump controls the mass flow rate it produces.
Furthermore, the additional fluid pumping mechanism of the ALHP manages the vapor buildup in the reservoir by removing a predetermined amount of vapor from the reservoir and transporting it to a secondary condenser for heat rejection. As a result, the ALHP can start up quickly and operate reliably like a generic LHP but with the additional capability of temperature control like a CPL. Active removal of vapor buildup in the ALHP reservoir by the auxiliary pump enables the system to operate in severely adverse conditions in which a CPL or an LHP cannot operate. For example, the ALHP can operate in a hot surrounding, the temperature of which is much higher than that of the ALHP without the need for an external thermal shielding mechanism that the CPL and LHP require.
According to an embodiment of the present invention, a heat transfer device includes a reservoir containing a working fluid and a porous wick for transporting the fluid through a closed loop system. It further includes an evaporator capillary pump for conducting heat from an outer surface to the wick inside, changing the state of the working fluid from liquid to vapor. A capillary link 210 between the evaporator capillary pump and the reservoir supplies liquid in the reservoir to the wick of the evaporator capillary pump. An auxiliary pump manages vapor buildup in the reservoir. A primary condenser condenses vapor from the evaporator capillary pump back to liquid state.
A secondary condenser may be implemented as a stand alone condenser or as part of the primary condenser to condense vapor from the reservoir back to liquid state. For cryogenic applications, a swing volume and a pressure reduction reservoir may be implemented to reduce system pressure and system weight.